Process for producing fibrous cement sheets

ABSTRACT

A process for producing cement sheets, particularly asbestos cement sheets, in which a cement mixture is fed into a rotating horizontal mould to form a soft centrifugally moulded cylinder, which is then removed from the mould on a mandrel, longitudinally divided, and laid out flat to harden. The mandrel both supports the cement cylinder during removal from the mould, and allows this to be rolled off the mandrel onto a receiving surface by rotating the mandrel and moving this transversely over the surface. Preferably, the cylinder of cement is formed within a liner in the mould, the edges of the liner being connected together during removal of the mandrel from the mould so that the liner assists in holding the cylinder on the mandrel. The invention also covers novel cement sheets produced by the process.

United States Patent H Barratt *July 1, 1975 [54] PROCESS FOR PRODUCINGFIBROUS 2,348.804 5/l944 GeriIty 162/[20 3,0l4,835 l2/l96l Feig ey etal. l. 162/l54 CEMENT SHEETS 3,097,080 7/!963 Weir 161/205 [76]Inventor: William C. Barratt, 595 Shepherd 3 773.44! H973 Banal: r l A H425/86 St., Niagara Falls, Ontario, Canada 3,778,206 l2/l973 Barratt425/86 Notice: The portion of the term of this Pat t ubs q n K0 1386- ll990, Primary ExaminerS Leon Bashore has b en dis laim d. AssistantExaminerPeter Chin [22] Filed: Apr. 20, I973 Related US. ApplicationData A f d t h t f l I process or pro ucmg cemen s ee 5, par lcu ar y[63] gy ssgfggxg of asbestos cement sheets, in which a cement mixture isfed into a rotating horizontal mould to form a soft centrifugallymoulded cylinder, which is then removed [30] Foreign Apphcanon PnontyData from the mould on a mandrel, longitudinally divided, July 3i, 1970Canada 89700 and out flat to harden. The mandrel both pp the cementcylinder during removal from the mould, [52] 162/120; 162/127? 162/13]:and allows this to be rolled off the mandrel onto a re- I62/154 162/155162/181 ceiving surface by rotating the mandrel and moving [62/230'162/384 this transversely over the surface. Preferably, the cyl l5 Cl. lin of cement s f d a liner in [he mouldv {58] or Search 162/120 f theedges of the liner being connected together during 2 removal of themandrel from the mould so that the 264,!08 425/84 86 liner assists inholding the cylinder on the mandrel.

The invention also covers novel cement sheets pro- [56] References Cltedduced by the process UNITED STATES PATENTS Ledeboer 162/]20 7 Claims, 20Drawing Figures nnnnnnnnnrmaanoaaoaa dannnnn nomaaoqagmpnnnnnnn FIG. IO

warwmux m5 SHEET FIG. l6

1 PROCESS FOR PRODUCING FIBROUS CEMENT SHEETS This application is acontinuation in part of my United States Pat. Application 166,778 filedJuly 28, 1971 now abandoned.

The present invention relates to a method for producing cement sheets,particularly asbestos fibre reinforced cement sheets, which may be flator curved or contoured. The invention makes use of apparatus moreparticularly described in co-pending US. Pat. Application Nos. 320,768now US. Pat. No. 3,778,206 and 320,770, filed Jan. 3, 1973. These sheetsmay be used for example as structural members, wall panels, or roofsections, or may be cut to smaller sizes to form tiles, mill work orshingles.

In the so called wet process for manufacturing asbestos cement sheets,for example as described in US. Pat. No. 1856570 to Ledeboer, a largeplant is required which, in order to be economical, must operate on ahigh production basis, and such a process is not easily adapted forproducing small quantities of special decorative materials.

The present invention provides a process which is suited to theeconomical production of small quantities of cement sheets, and whichcan readily be varied to produce different surface finishes orthicknesses in the sheets. The process can easily be modified, so that awide variety of different cement sheets can be produced in a short time.The apparatus used for the invention does not require a large investmentor take up much space, and is suitable for location close to where thebuilding is taking place. Furthermore, the nature of the process gives ahigh strength product due to avoidance of excess water, uniformity ofwall thickness, high density and predetermined directional resistance tostresses. Preferably, the cement is reinforced by fibres, and in thisprocess additional strength is given by good orientation of fibres.

Specifically, the process produces a cement sheet having a mat of fibreswhich, although having a preferred orientation in the plane of thesheet, also extends continuously across the major part of the totalthickness of the sheet. Sheets produced in accordance with the inventionare thus distinguishable from sheets produced by simple mouldingprocesses such as are described in US. Pat. No. 3,0l4,835 to Feigley etal, and US. Pat. No. 3,634,562 to Kole et al, and which have nopreferred orientation of the fibres. Furthermore, the sheets of thisinvention are also distinguishable from sheets produced by processes inwhich a number of layers of material are built up for example on thesurface of a cylinder, as described in the aforementioned US. Pat. No.1,856,570, and also as described in US. Pat. No. 1,893,783 to Moeller.The sheets produced in accordance with these methods are somewhat likecardboard in that they have a number of distinct laminations each with adistinct fibre mat. In this connection, British Pat. No. 268,ll toCarnes is also of interest; this described cement sheets formed byfeeding the cement mixture intermittently into a spinning cylindricalmould to form a series of layers or plies. The Carnes method would givean inferior product, since when each batch of material had been fed inthis would form a layer with the light gypsum component on the insidesurface of the layer, so that the final product would have layers ofgypsum into which water could seep when the product is exposed, causingde-lamination.

In accordance with one aspect of the invention, a process for producinga cement sheet comprises the steps of:

a. feeding a cement mixture into a rotatable horizontal cylindricalmould,

b. spinning the mixture in the mould, distributing the mixture aroundthe mould walls, and removing water therefrom, to form a soft, hollowcement cylinder,

c. inserting a cylindrical mandrel into the said cement cylinder toengage the inner surface of said cylinder,

d. sliding the cement cylinder from the mould without dimensional changeby withdrawing the mandrel while holding the cylinder on the mandrel andsupporting the inner surface of the cylinder by means of the mandrel,

e. dividing the cement cylinder longitudinally, and

f. progressively removing the cement cylinder from the mandrel as asheet by initially placing one edge of the sheet on a receiving surfacedisposed beneath the mandrel and subsequently rolling the cement sheetoff the mandrel by simultaneously rotating the mandrel and causingrelative movement between the mandrel and the said receiving surfacetransverse to the mandrel axis.

The term rotatable horizontal cylindrical mould" means a cylindricalmould which is rotatable about a horizontal axis.

The term mandrel as used herein will be understood as meaning merely acylindrical member having a diameter suitable for directly engaging theinner surface of the cylinder formed in the mould, without substantialdimensional change in the cylinder. The use of a mandrel is importantfor giving support to the soft cement cylinder during removal from themould and during any slitting step.

The expression dividing the cement cylinder longitudinally" includesboth slitting the cement cylinder longitudinally, while this is held onthe mandrel, or alternatively merely splitting the cement cylinder alonga longitudinal discontinuity previously formed in the cylinder, forexample by means of an inwardly projecting member extendinglongitudinally of the mould. The longitudinal discontinuity may be acomplete division, as will be the case where the radial dimension of thein wardly projecting member is greater than the radial thickness of thecement deposited in the mould.

The soft cement cylinder formed within the mould may have one enddefined by a member movable axially of the mould, the process furtherincluding the steps of displacing the axially movable member from theone end of the cylinder prior to final positioning of the mandrel withinthe cylinder, and substantially expanding extracting means connected tothe mandrel into the space previously occupied by the axially movablemember. The extracting means are maintained in the expanded condition toprevent axial movement of the cylinder relative to the mandrel duringthe step of withdrawing the mandrel from the mould.

The term movable axially denotes that the member can be moved with anaxial component of movement to provide a space around the whole end faceof the cement cylinder; this does not however preclude the member frombeing pivotally mounted, as for example is the removable end plate atthe other end of the mould.

The process of this invention preferably involves the use of a liner;this helps to hold the soft cement cylinder on the mandrel during itsremoval from the mould, and also allows an outer facing, for example anaggregate facing, to be held securely in place on the outside of thecement cylinder. The preferred process thus includes a preliminary stepof inserting a cylindrically curved flexible sheet of material into themould so that the mate rial forms a liner conforming to the mouldsurface and has adjacent edges extending longitudinally of the mould,and the subsequent steps of holding the edges of the liner togetherwhile withdrawing the mandrel from the mould and disconnecting the lineredges before dividing the cement cylinder longitudinally. The liner maybe prevented from moving axially of the mandrel by extracting meansexpandible into the space between the end of the cement cylinder and theaxially movable member, and which engage the end of the liner.

The use of extracting means which are expandible into the spacepreviously occupied by an axially movable member is not howeveressential, since a liner similar to that disclosed herein could beprovided with special means engageable by extracting means on themandrel. For example a flange at either end of the liner, or holes orhooks at that end of the liner adjacent the end of the mould throughwhich the cement cylinder is removed, could be engaged by extractingmeans carried by the mandrel; the extracting means may be hooks.Alternatively, the liner may have inwardly projecting longitudinalflanges with holes therein, which holes are engageable by hooks spacedalong the mandrel. Specific apparatus of this type is shown in maycopending U.S. Application No. 320,768, in FIGS. 7 to 12. in accordancewith a further aspect of the invention, therefore, a process forproducing a cement sheet comprises the steps of:

a. inserting a cylindrically curved flexible sheet of material into arotatable horizontal cylindrical mould so that the material forms aliner conforming to the mould surface and has adjacent edges extendinglongitudinally of the mould,

b. feeding a cement mixture into said mould and spinning the mixture inthe mould to form a cement cylinder having its outer surface defined bysaid liner,

c. inserting a cylindrical mandrel into the cement cylinder to engagethe inner surface of said cylinder,

d. moving extracting means connected to the mandrel into position toengage said liner and to cause said liner to move with the mandrel whenthe mandrel is withdrawn,

e. sliding the cement cylinder from the mould by withdrawing saidmandrel while holding said liner edges together,

f. disconnecting the adjacent edges of the liner and dividing the cementcylinder longitudinally, and laying out the resultant cement sheet onthe liner.

The edges of the liner may be held together by clip means attached tothe edges before the liner is inserted into the mould.

Also, it is desirable to use an expanding mandrel, described in moredetail below, which is expanded into contact with the interior surfaceof the cylinder after insertion into the mould.

It is preferred that the step of inserting the mandrel into the mouldshould be performed while the mould is still spinning, since otherwisewith large sizes of cement cylinders, there may be a danger of collapseof the cylinder when the mould is stopped. The mandrel may be caused tospin at the same speed as the mould before contact occurs between themandrel and the cement surface, by use of interengaging means on themandrel and the mould.

The centrifugal process used in accordance with the invention isparticularly useful for producing laminated sheets, since a materialdistinct from the cement mixture may be fed into the mould at apredetermined stage in the feeding process, and tends to become evenlyspread out, forming a uniform layer, and the centrifugal forces bond thesuccessive layers strongly together. Accordingly, laminated sheetsproduced in accordance with the invention are characterized by layers ofuniform thickness which layers are intimately and strongly bonded toeach other without the need for any bonding agent. in addition, theprocess as described produces a product which is strong by virtue of thesmall amount of water used during the spinning step.

The invention preferably makes use of a cement mix which includesasbestos fibres. These fibres and other reinforcing fibres becomeorientated predominently parallel to the surface of the mould, and thusare predominently orientated in the plane of the finished cement sheet.In addition, depending on the speed at which the mould rotates, thefibres may become preferentially orientated circumferentially of themould, so that in the final cement sheet the fibres have a preferredorientation in one direction. The term preferred orientation" means thata majority of the fibres are orientated at angles to one direction, thatof preferred orientation, which are smaller than the angles which suchfibres make with directions perpendicular to the preferred orientation.

Although the fibres will tend to be orientated in the plane of thesheet, there is sufficient deviation from this orientation that cementsheets produced in accordance with the present invention have acontinuous mat of reinforcing fibres incorporated therein which willgenerally extend across the thickness of the sheet. It may be, however,that asbestos is fed in only during a part of the feeding process, butin any case the cement sheet will generally have a continuous matextending across the main part of the thickness of the sheet. Asexplained above, it is this characteristic which distinguishes sheetsmade in accordance with this invention from sheets made by the so-calledwet process" having separate laminations. The product given by theprocess of this invention has a modulus of rupture in bending of 5,500pounds per square inch, as compared with about 4,500 pounds per squareinch for products made by the above-mentioned wet process," and amolulus of rupture in bending of at least 5,000 pounds per square inchcan be expected.

The different materials which go to form the laminated sheet may becements of difi'erent compositions, or different colours; or one of thelayers may be formed of a particulate material which becomes bonded tothe cement layer. In this manner, cement sheets having surfaces ofexposed particles, for example of aggregate material or glass, metal, orplastics materials, may be produced. ln the case of these products, aswell as the sheets having different cement laminations, the layers areintimately bonded together; i.e. held together by the cement of thelayers themselves rather than by bonding agents which are otherwise usedto bond particles onto cement sheets.

The process will be particularly described by way of example withreference to the accompanying drawings in which:

FIG. 1 is a partly sectioned elevation of a first type of centrifugalmould for use in accordance with the invention,

FIG. 2 is a cross-section through the mould of FIG.

FIG. 3 is an elevation of a mandrel,

FIG. 4 is a cross-section through the mandrel on line 4-4 of FIG. 3,

FIGS. 5 and 6 show details of the mandrel in crosssection and sideelevation respectively,

FIG. 7 is a sectional elevation through the mould with the cylinder andmandrel in place, caused FIG. 8 is a view of the mandrel in the laststage of the process,

FIGS. 9 and 10 show variations of the basic method which produce acement sheet with an exposed particulate surface,

FIGS. 11 and 12 are edge views of special cement sheets produced inaccordance with the invention,

FIG. 13 is a partly sectioned elevation of a second type of centrifugalmould for use in accordance with the invention.

FIG. I4 is a cross section on lines l414 of FIG. 13 showing details ofliner locking means.

FIG. 15 is a partly broken away view of a modified mandrel for use withthe mould of FIG. 13,

FIG. I6 is a cross-sectional view of the modified mandrel on lines 16-46of FIG 15,

FIG. 17 is a partly sectioned view of the mould of FIG. 13 with themodified mandrel in place, and

FIGS. 18a, 18b and 18c illustrate modified apparatus for slitting,laying out and curing the cement sheets.

Referring to FIGS. 1 and 2, the apparatus includes a rotatablecentrifugal mould indicated generally at I0. The mould has a cylindricalwall 12, which is of sheet metal and is perforated with l/l6 inchdiameter holes I2a spaced apart on one inch centres circumferentiallyand axially of the mould; these holes allow outflow of water duringspinning. The mould wall 12 is reinforced by an end ring 14 securedwithin one end of the wall, and which is ofa thickness greater than thecement cylinder to be spun; and also by reinforcing rings or tires 15which rest on four rollers 16. The rollers 16 are carried by twoparallel horizontal shafts 17, supported in bearings 18, and one ofthese shafts is driven by a motor and gearbox combination 19. When themotor is operated to drive the shaft I7, the frictional contact betweenrollers 16 and the tires 15 causes the mould to rotate about itshorizontal axis.

the mould has a fixed end plate 21 and a removable end plate 22. Thefixed end plate 21 is of annular form having a central aperture 2lawhich is large enough to allow fluid cement material to be fed into themould by feeding means in the form of a charging head 23 to bedescribed. The fixed end plate 21 is held by nuts on threaded studs 21bwelded to the mould wall, and is removed only for maintenance or mouldalterations.

The removable end plate 22 is mounted on an axially projecting stubshaft 26, which is carried in a bearing 27 supported at the end of aswing arm 28. The swing arm is pivoted at 30 and is rotatable from theclosed position of FIG. I to the open position of FIG. 7 by means of apower cylinder 31. The inside of end plate 22 is provided with a centrallocating ring 33 having an inner surface which flares outwardly towardsthe other end of the mould, and which serves to support and locate oneend of the charging head 23.

It will be apparent that the end plate 22 need not entirely cover theend of the mould, but may instead provide a cover over a annular portiononly, of radial thickness greater than that of the cylinder to be spuntherein.

The charging head 23, as shown in FIG. 1, is mounted on a supportcarriage 24 mounted on rails 24a adjacent that end of the mould with thefixed end plate. These rails allow movement of the charging head fromthe operative position of FIG. I, in which the charging head extendsthrough the fixed end plate 21, to a retracted position in which thecharging head is withdrawn clear of the mould.

The charging head comprises a horizontally extending support membersomewhat longer than the mould in the form of a parallel sided channel34 open at its lower end and reinforced at its top by a horizontal plate34b. At its outer end (remote from the carriage 24) the plate 34bcarries a positioning pin 35, which, when the charging head is in theposition of FIG. 1, locates in the locating ring 33 and assists insupporting the outer end of the charging head. The support membercarries at one side a cement feed pipe 36, having downwardly extendingdistribution pipes 360 spaced along the pipe 36 at 1 foot spacing. Belowthe lower ends of pipes 360 there is provided a curved shutter 38 which.when in the closed position shown in broken lines, co-operates with theoutside of the channel 34 to provide a trough suitable for retaining aquantity of particulate material. The shutter 38 is mounted on radialarms 39 pivoted to the support member at and pivotally movable relativeto the support member by fluid cylinder 81 to move this shutter into theopen position shown in full lines, allowing material held in the shutterto be released onto the inside of the mould or to be fed directly intothe mould from discharge pipes 36a.

The channel 34 accommodates a hollow parallel sided member 83 verticallyslidable therein under the control of hydraulic cylinder 84. The lowerend of member 83 has a curved corner joining the vertical side of themember 83 with the horizontal lower edge thereof, this curved cornerfacing the direction of rotation of the mould which direction isindicated by the arrow. This curved corner and the adjacent surfaces ofmember 83 form spreader means 86 having a hard facing and capable ofcontacting the inner surface of a hollow cylinder of material being fedinto the mould, to assist in evenly spreading the material within themould. During the stage in which material is being fed into the mouldvia pipes 36a, the spreader means is carried to rise at a steady rateunder control of fluid cylinder 84.

The member 83 also incorporates means for removing water from within thecentre of the cylinder of material being spun within the mould, in theform of a skimmer 88 which is movable of fluid cylinder 89 between aretracted position in which it does not extend below the surface of thespreader means, to an extended position as shown in broken lines inwhich it projects below the lower surface of the spreader means. Theskimmer 88 has an opening facing the direction of rotation of the mould,and which communicates with a water discharge pipe 90 extending alongthe member 83.

The member 83 also incorporates a vibrator 92 mounted on the inside ofthe spreader means and arranged to cause vibration of the spreader meansfor compacting material in the mould.

The apparatus also includes a special cylindrical mandrel 40, which isshown in FIGS. 3 to 7. The man drel has a cylindrical outer wall 400,the diameter of which is selected so that this can be inserted into acement cylinder which has been formed in the mould. The diameter of themandrel is such that the cylindrical surface of this is engageable withthe inside surface of the cement cylinder without dimensional change ofthe latter. An outer end of the mandrel is closed by an end plate 41(see FIG. 7), to the centre of which is welded a guide member 42 in theform of a tubular shaft. The guide member 42 is longer than the lengthof the mould, so that it can be used to guide the mandrel into themould. The other end of the mandrel has a shorter shaft 42a projectingaxially therefrom.

The mandrel wall 40a is pierced by longitudinal rows of ports 43, andeach row of ports communicates with a longitudinal channel 44 within themandrel. These longitudinal channels, which are shown in cross-sectionin FIGS. 4 and S, are constituted by channel members 45 having theirouter edges welded to the inside of the wall 40a, each row of ports 43being centrally positioned along a channel member. FIGS. and 6 showvalve means 47 which allow one end of each channel 44 to be individuallyplaced in communication selec tively with a vacuum header tube 48 or acompressed air header tube 49. These header tubes 48 and 49, which areshown in FIGS. 5 to 7, are toroidal tubes of equal dimensions positionedclose together near to one end of the mandrel, and connected by radialconduits S0 and 51 and elbow connectors respectively to flexible hoses52 and 53. The hoses 52 and 53 serve to connect the header tubes 48 and49 to sources of vacuum and pressurized air respectively, while allowingfor rotation of the mandrel by at least one revolution.

The valve means 47, one of which is provided for each of the channels44, comprises a valve body 55 in the form ofa rectangular block havingits inner surface supported by the header tubes 48, 49 and having itsouter surface fixed to a channel member 45. The valve body has tworadial bores 57 and 58 leading respectively from header tubes 48 and 49into the channel 44, and a valve member 59 is slidable in an axial borein the valve body to control flow of air through these radial bores. Thevalve member 59 has a central recessed portion 60', in the positionshown in FIG. 6 this recessed portion is between the radial bores 57 andS8 and the valve member prevents flow of air through either bore Bypulling out an extension 61 ofthe valve member, the recessed portion 60is brought into registry with the bore 57, so that the channel 44 is putinto communica tion with the source of vacuum through header 48 and hose52', and when the valve member is pushed inwardly to bring recessportion 60 into register with bore 58 then the channel 44 is placed incommunication with the source of compressed air via bore 58, header 49,and hose 53.

A curved operating member may be rotatably mounted at the outer end ofthe mandrel. and arranged to engage the valve members as it rotatesrelative to the mandrel to move these from the vacuum position to thecompressed air position.

FIG. 7 is a view of the mould with a spun cement cylinder in place, andwith the mandrel inserted to contact the inner surface of the spuncement cylinder, which is indicated at 70. it will be seen that themandrel is an easy clearance fit within the cement cylinder, and thatthe end of the mandrel fits within the reinforcing ring 14.

The apparatus also includes mounting means for the mandrel which may bein the form of known handling means such as a small factory crane orhoist, and which supports the mandrel by its axial shafts 42 and 42a.The mounting means must allow axial movement of the mandrel into themould, and must also allow rotation of the mandrel at least almost 360with relative transverse movement between the mandrel and the surfacewhich receives the cement sheet, for releasing and unrolling the cementsheet from the mandrel in the manner to be described.

The only other essential parts of the apparatus are slitting means forforming a longitudinal slit in the cylinder of soft cement held on themandrel, after removal from the mould; means for causing relativemovement between the slitting means and the mandrel to form thelongitudinal slit, and a surface onto which the cement sheet can be laidafter removal from the mandrel.

In the simplest apparatus, the slitting means may be a blade, such asblade 73 of FIG. 8, projecting upwardly in fixed position, andassociated with the receiving surface 72, the blade having a length atleast equivalent to that of the mandrel and having a width greater thanthe thickest sheets to be produced. in this embodiment, the means forcausing relative movement between the slitting means and the mandrelwill be the handling or mounting means of the mandrel which allow thisto be lowered onto the blade 73. The receiving surface may be providedby a pallet which can be located suitably in relation to the slittingblade for receiving a cement sheet removed from the mandrel, and whichis then removable to another location where the cement sheet hardens.The receiving surface 72 may be a plan flat surface or curved, or mayhave a relief design thereon for producing a corresponding relief designin the cement sheet. In the latter case, the apparatus will also includea standard type of hydraulic or other press by means of which pressurecan be applied to the top surface of the cement sheet to cause the lowersurface to assume the shape of the pallet surface.

The process of the invention will now be particularly described withreference to FIGS. 1 to 8.

The charging head 23 is firstly advanced into the mould on rails 24a, tothe position of FIG I, and the removable end plate 22 is closed. Thecylinder 84 is actuated to bring the spreader means 86 close to themould surface, and cylinder 81 is operated to open the shutter 38 to theposition shown in full lines. A cement mix including reinforcing fibresis then fed onto the mould surface via the pipes 36 and 36a, andsimultaneously the cylinder 84 is operated to raise the spreader means86 gradually, so that these maintain contact with the cement mix andensure that this is spread evenly within the mould. The cement mix isfed in while wet (with about water) and rotation of the cylinder at aperipheral speed of 1200 ft/min. for about five minutes caused the wetcement to form a cylindrical layer of uniform wall thickness on theinside of the mould. The centrifugal effect also concentrates andcompacts the solid particles of the mix in the outer parts of the cementlayer, and excess water which does not flow out of the perforations inthe wall of the mould is forced to the center of the mould and flows outof this during a de-watering stage.

The use of a cement mix containing only 70% water is in contrast toother methods where more than 90% excess water is required.

The de-watering stage is reached after the material has spunsufficiently to compact the solids into cylindrical form and to form aninner layer of excess water. This takes about one minute. At this stagethe cylinder 89 is operated to cause skimmer 88 to protrude from thelower surface spreader means, and cylinder 84 is operated to move themember 83 downwardly until the skimmer contacts the layer of waterwithin the mould. The water is then skimmed off by pumping this throughdischarge pipe 90. In the final stages of de-watering, the spreadermeans 86 is brought into contact with the ce' ment cylinder, and thevibrator 92 is activated to cause vibration of the spreader means, andcompaction of the cement, while the mould continues to spin.

At this stage rotation of the mould is stopped, and the end plate 22 isremoved to the position shown in FIG. 7 by operation of cylinder 31. Thecylindrical mandrel 40 is then positioned in the mould, by suitablehandling means, into the position of FIG. 7.

When the mandrel is in place (as shown in FIG. 7) the valve members 61are operated to connect the channels 44 to the source of vacuum so thatthe cement cylinder, which is already contacting or almost contactingthe mandrel surface, is forced against the surface by atmosphericpressure. The mandrel is then withdrawn from the mould, and subsequentlythe cement cylinder is slit by being lowered, while still on themandrel, onto the blade 73 so that this slits the cylinderlongitudinally. The mandrel carrying the cement cylinder is then rolledalong, ie., moved transversely over the surface 72 while beingsimultaneously rotated, so that the cement sheet is laid out on thesurface 72 (FIG. 8). During removal of the cement sheet the valvemembers 61 are operated successively to cause the lowermost channels 44of the mandrel (eg. channels 44a and 44b of FIG. 8) to be connected to asource of positive air pres sure to assist the removal of the cementsheet from the mandrel. The valve members 61 of channels from which thecement sheet has been removed are returned to the neutral position ofFIG. 6.

Preferably, the cement sheet so produced is subjected to a finalpressing operation as between the platens of a press, this givinggreater strength and density to the product. Subsequently, the cementsheet is hardened by curing in known manner. Also, the cement sheet maybe rolled off the mandrel onto a platen hav ing a relief design, thecement sheet being pressed onto the platen by the mandrel to reproducethe design.

It may be noted that the mandrel not facilitates removal of the cementcylinder from the mould, and the slitting of this, but also enables thecement sheet to be rolled and compacted while being laid out.

Where the reinforcing fibres are to be asbestos fibres, the cement mixis produced by opening asbestos of desired grade to separate the fibresand mixing these fibres with Portland cement, and if desired, silicasand flour. Water is added to give a mixture which is approx imately 30%solids and 70% water. The mixing is thor ough so that the maximum numberof fibres are coated with cement and water. When this mixture is fedinto the rotating mould, the asbestos fibres tend to be laid more orless fiat, i.e. substantially parallel to the cylindrical surface of themould, giving a laminated cement sheet which has greater strength thanone in which the fibres are randomly oriented. As explained, the fibresform a mat which extends across the whole thickness of the cement sheet,apart from any particulate or special surface. Furthermore, the fibresalso have a tendency to become orientated in the circumferentialdirection of the mould and thus give a cement sheet in which the fibreshave a preferred orientation in this direction. Such cement sheets haveenhanced properties in this direction, making it possible to designsheets which have strength properties suited for particular purposes.The circumferential orientation of the fibres can be varied somewhat byvarying the speed of rotation of the mould. For example, spinning at aperipheral speed of l200 ft. per minute is suitable where it is merelyrequired that the fibres be parallel to the surface. Faster spinningwill tend to orientate the fibres circumferentially; for example at 2000ft. per minute more than half the fibres would be orientatedsubstantially circumferentially.

Clearly, any of the cement sheets made according to the processesdescribed can be cut to desired forms, to give tiles or other products.If necessary, cutting can be carried out before the cement hardens.

laminated cement sheets may be produced in accordance with the inventionby feeding a second material, distinct from the cement mixture, into themould at a predetermined stage in the feeding process. The manner inwhich different materials are fed into the mould will vary widelyaccording to the desired laminating effects. The material used may allbe cement mixes, but of varying compositions or colours. For example, inone process, a fine coloured cement mix is first fed into the rotatingmould, and the mould is spun until this cement mix has formed a layer ofuniform thickness on the interior of the mould. Next, a coarser concretemix is fed in while the mould continues to spin to form a furthercylindrical layer within the layer of coloured cement, the concretelayer being thicker than the coloured cement layer. The process thenproceeds through the spinning and de-watering stages as described above,and the cement cylinder is then removed from the mould, slit, and laidout in the form of Y a cement sheet. The cement sheet so formed islaminated, having an outer facing layer of fine, coloured, cement, and athick supporting layer of concrete.

In another process in accordance with the invention, the material firstfed into the mould is a particulate ma terial mixed with cement andwater or any suitable bonding agent. The particulate material may beparticles of stone having an average size between /2 inch and 1/64 inch,or particles of glass, or metal, or of plastics material. As before,this particulate material is fed in while the mould is rotating, androtation is continued until the material has formed a uniformcylindrical layer within the mould. Subsequently, a cement mix is fedinto the mould, and the mould is spun to produce a flaccid laminatedcement cylinder having the particulate material bonded to the outerlayer. The cement cylinder is removed from the mould on the vacuummandrel, as described above, slit, and the cement sheet laid out toharden. This process produces a product having a first layer with acontinuous mat of fibres, as described above, and a second layerincluding a matrix of cement which is integral with the cement of thefirst layer and in which is embedded the particulate material. Onesurface of the cement sheet may then be polished to provide a flatsurface in which the particulate material is exposed.

Alternatively the particulate material may be exposed, as in theso-called exposed aggregate surface, by removal of material surroundingthe particles. In this case, a chemical retardant may be applied to thesurface of the cement sheet on which the particulate material isconcentrated. This can be done by applying the retardant to the palleton which the cement sheet is laid out. After the main part of the cementsheet has hard ened, the layer having the retardant is still soft, sothat the cement in this layer can be brushed away to expose theparticulate material. Suitable chemical retardants for this purpose,which retard the setting of cement, are well known in the art. A cementsheet may be produced by this process which the surface particles are inintimate contact with the cement, i.e. are held onto this without theuse of any bonding agent, which may deteriorate with agev The shutter 38provides convenient means for supplying the particulate material to theinner surface of the mould. Before the charging head 23 is advanced intothe mould, the shutter 38 is closed by cylinder 81., and the troughformed between shutter 38 and the side of member 34 is filled with theparticulate material. After the charging head is in position in themould the shutter 38 is opened to release the particulate material ontothe inner surface of the mould. This material is then evenly spread inthe mould by operation of the spreader means 86. Subsequently, a cementmix is fed into the mould, and the mould is spun to produce a flaccidlaminated cement cylinder having the particulate material bonded to theouter layer. The cement cylinder is removed from the mould on the vacuummandrel, as described above, slit, and the cement sheet laid out toharden. The surface of the cement sheet may then be polished to providea flat surface in which the particulate material is exposed.

Instead of being a plain flat surface, e.g., the surface of a pallet,the receiving surface may be such as to be capable of becoming bonded tothe soft cement sheet. One example of this is illustrated in FIG. 9,which shows a soft cement sheet being laid out onto a bed of aggregatematerial. Bonding of the soft cement sheet to the aggregate material isachieved firstly by using the mandrel in the manner of a roller, andthen by pressing the resultant product between the platens of a press.The final product is illustrated in FIG. 11. Instead of using a bed ofparticulate material, the soft cement sheet can be laid out on a sheetalready prepared according to the process, so that a thick laminate canbe built up.

In the process of FIG. 10, an asbestos cement sheet which has beenproduced in accordance with the invention, as described, is laid outflat on a pallet, but before the sheet hardens aggregate material issprinkled on the sheet, and pressed in, and the sheet is then allowed toset.

FIG. 12 shows a portion of a cement sheet having a sculptured or reliefdesign surface. This can be made by laying out a soft sheet onto apallet having a corresponding relief design surface, and pressing thesoft sheet into place by means of a press.

Also, instead of the mandrel being moved transversely over the receivingsurface, the surface may be arranged on a carriage for movementunderneath a stationary, but rotatable, mandrel.

Additionally, cement sheets with liners bonded thereto may be producedby inserting a liner into the mould before adding the cement, so thatthe cement cylinder becomes bonded to the liner. The bond can bestrengthened by chemical bonding with a vinyl acetate bonding agent suchas the aforementioned Albitol, applied to the inner surface of theliner. A gap may be left between the adjacent edges of the liner, toallow for slitting by the blade 73. Alternatively the liner itself mayform the longitudinal division in the cement cylin' der as particularlydescribed in co-pending Pat. Application No. 320,768. Liner materialsmay be of metal, plastics or rubber.

The liners used may be plain and imperforated, in which case anon-perforated mould would be used and all the excess water would beremoved from the centre of the hollow cylinder produced. A plain linermay be used with the vacuum mandrel described or with the differenttypes of apparatus shown in FIGS. 13 to 18.

The cement sheets produced in accordance with this invention can befurther treated by drying when hard at temperatures of 105C or higher,soaking in a liquid monomer such as methyl methacrylate, and curing itto polymerize the monomer. Polymerization can be achieved by irradiatingthe impregnated cement or by heating the cement to 75C for 2 hours with2% benzoyl peroxide added to the monomer as a catalyst. This processgives much improved physical properties to the cement sheet, and alsomakes it impermeable.

A different form of apparatus for performing the same basic process, butin which the mandrel is fully rotatable when inserted in the mould, willnow be described with reference to FIGS. 13-18.

FIGS. 1 and 14 show moulding apparatus generally similar to that ofFIGS. 1 and 2, but adapted for use with the modified form of mandrelshown in FIGS. 15

and 16.

The apparatus of FIGS. 13 and i4 is similar to that previously describedin having a rotatable mould with a cylindrical wall 112 held by ringsI15 resting on roolers 116 mounted on shafts 117 driven by a motor andgearbox combination I19. Unlike in the first embodiment, however, themould wall 112 is not perforated. The dimensions ofthe mould are about10 ft (3 meters) length and about 6 ft (1.8 meters) in diameter. The

mould has a removable end plate 122 which again is the same as that ofthe first embodiment. At the end of the mould opposite the plate 122however the construction is different from the first embodiment, in thatit incorporates an axially movable annular member 121 hereinafterreferred to as a void ring, which will now be described.

The void ring 122 is of channel form, having an outwardly facing openingengaged by an annular plate which is bolted to the end flange of themould. The void ring is movable axially of the mould between thepositions shown in FIGS. 13 and 17, and sealing means 18] provide a sealbetween the inner flange of the ring and the inner surface of the mould.The void ring 121 has a radial dimension greater than that of thethickest cylinders to be spun within the mould, and has interiordimensions large enough to allow space for entry of the charging head123 which is identical to the charging head 23 already described withreference to the first embodiment, and which is shown in cross sectionin FIG. 14.

FIG. 14 also shows a special form of liner 170 used in this secondembodiment. This liner is a sheet of abrasion resistant rubber of /2inchto 1 inch thickness sufficiently flexible to form a cylinder lining themould as shown. One edge portion of the liner is preferably thickened toprovide an inwardly projecting lip surrounding that end of the lineradjacent the void ring 121. The liner has opposite edge portions whichare adjacent while the liner is in the mould, the edge portion beingbonded to sheet metal edge pieces 171. The edges of the liner extendbeyond the edge pieces 171 to form a seal preventing cement or othermaterial from passing between the edge pieces 171. The edge pieces 171are provided with radially outwardly directed flanges 172, and the outeredges of these flanges have lips 173 bent back parallel to the linersurface and forming a recess therewith. These recesses are engageable bythe inwardly directed jaws ofa locking clip 175, the cross-sectionalform of which is shown in FIG. 14, and which is accommodated in asuitably shaped longitudinal groove 176 in the mould surface. The clip175 thus co-operates with the edge pieces 171 to hold the edge portionsof the liner together when the cylinder and liner are removed togetherfrom the mould, as will be described, thus holding the cement cylinderon the mandrel when removed from the mould. To facilitate removal fromthe mould, the outside surface of the liner, and the inside surface ofthe mould, are coated with polytetrafluoroethylene.

The modified mandrel 140 used with this mould is an expanding mandrel asshown in FIGS. and 16. The mandrel includes a cylindrically curvedsurface formed by a slightly flexible metal plate 141, the longitudinaledges of which are close together when the plate 141 is unstressed andis at its minimum diameter as indicated in full lines in FIG. 16. Theplate 141 has an annular end flange 142 at one end, and is capable ofbeing expanded by means to be described to the condition shown in brokenlines in FIG. 16, the amount of radial expansion being greater than theradial depth of flange 142.

The mandrel is mounted on a tubular shaft 144 carried at its inner endby a rail mounted carriage 148, the carriage allowing axial movement ofthe mandrel into the mould and also allowing free rotation of the shaft144. The shaft 144 has an extension 145 at the outer end of the mandrel(i.e., that remote from the carriage 148). which serves a purpose to bedescribed. The shaft 144 carries two axially spaced expandingarrangements 151 and 152 by which the plate 141 is connected to theshaft 144. Each expanding arrangement comprises two opposed pairs ofgussets 154 welded to shaft 44, and two symmetrically arrangedtriangular crank members 155 each having a first corner pivoted at 156to gussets 154, a second corner pivoted to gussets 157 which are weldedinternally to plate 141 near to the longitudinal edges thereof, and athird corner pivotally connected by links 158 to gussets 159. The crankmembers 155 of expanding arrangements 151 and 152 are connected formovement together by plate 155a. Also, both crank members 155 and links158 are of channel form, having parallel side plates or flangesconnected by a web. The

gussets 159 are welded to the inside of plate 141 at locations spacedaway from gussets 157, being on the opposite side of a plane throughpivots 156 from these gussets 157. The spacing between adjacent gussets157 and 159, is roughly equivalent to that between the two gussets 159,this spacing being of the order of to l20 of arc subtended at the axisof shaft 144. The two crank members are arranged to be urged apart by apneumatic cylinder 161, this cylinder being connected by a flexibleconduit 162 via a rotary valve 163 to an air space within the hollowshaft 144. The space within the shaft 144 is closed apart from theconduit 162 and a quick connector coupling by means of which a charge ofcompressed gas can be admitted into this space. The valve 163 is biasedinto a shut position, but is arranged to be opened by rotary movementwhich occurs when the valve arm 163a, extending radially from the insideend of the mandrel, is pivoted sideways by contact with an arm 164 whichrotates with the mould. Admission of pressurized air into cylinder 161is arranged to cause extension of this cylinder, with consequentoutwards movement of members 155 and expansion of the plate 141 into thebroken line position. The pivotal movement of valve arm 163a is limitedso that after the valve 163 has been opened by arm 164 of the mouldthese arms continue to engage with each other causing rotation of themandrel with the mould. The arms 163a and 164 are so situated on themandrel and mould respectively as to cause the mandrel to be rotatedwith the mould in such position that the adjacent longitudinal edges ofthe plate 141 forming the mandrel surface are disposed adjacent thejunction in the liner (see FlG. 18b), so that after expansion of themandrel a gap is provided in the cylindrical surface of the mandrelwhich gap underlies that between the flanges 172 of the liner.

The outer end of the mandrel is provided with a protruding spring loadedplunger 166, positioned to engage the void ring 121 so that this voidring is pushed away from a cement cylinder formed in the mould onpositioning of the mandrel in this cylinder.

in operation, liner 170 is bent into the form of a cylinder, and itsedges are connected together by clip 175, before the liner is fittedinto the mould as shown in FIG. 14. The end plate 122 is then placed inthe closed position, and the void ring 121 placed in the inner positionshown in FIG. 13. With the shutter 38 of the charging head 123 heldclosed by cylinder 81, the trough formed by this shutter is filled withaggregate material. The charging head 123 is then advanced into themould on rails 24a, to the position of FIG. 13. The cylinder 84 isactuated to bring the spreader means 86 close to the mould surface, andcylinder 81 is operated to open the shutter 38 to the position shown infull lines, thus dumping the aggregate material which distributes itselfwithin the mould. A cement mix is then fed onto the liner surface viathe pipes 36 and 36a, and simulta neously the cylinder 84 is operated toraise the spreader means 86 gradually as feeding proceeds. so that thesemaintain contact with the cement mix and ensure that this is spreadevenly within the mould. The cement mix is fed in while wet (with about70% water) and rotation of the cylinder at a peripheral speed of 1200ft/min for about five minutes causes the wet cement to form acylindrical layer 149 of uniform wall thickness on the inside of theliner. within an outer layer of aggregate. The void ring 121 defines theend of the cylinder 149 of cement remote from plate 122. The centrifugaleffect also concentrates and compacts the solid particles of the mix inthe outer parts of the cement layer, and excess water is forced to thecenter of the mould and flows out of this during a dewatering stage.

The de-watering stage is reached after the material has spunsufficiently to compact the solids into cylindrical form and to form aninner layer of excess water. This takes about l minute. At this stagethe cylinder 89 is operated to cause skimmer 88 to protrude from thelower surface of the spreader means, and cylinder 84 is operated to movethe member 83 downwardly until the skimmer contacts the layer of waterwithin the mould. The water is then skimmed off by pumping this throughdischarge pipe 90. In the final stages of dewatering, the spreader means86 is brought into contact with the cement cylinder, and the vibrator 92is activated to cause vibration of the spreader means, and compaction ofthe cement, while the mould continues to spin.

Once a hollow cement cylinder has been formed, and the dewatering andcompacting steps have also been performed as described for the firstembodiment, the end plate 122 is lifted clear (as in FIG. 17), the spacewithin the shaft 144 is charged with compressed air, and the mandrel 140is inserted into the cylinder, while the cylindrical mandrel surfaceformed by plate 141 is in its relaxed, contracted state, and while themould is still spinning, so that the cement cylinder is maintained incontact with the liner. Before insertion, the mandrel is preferablycaused to spin at a speed slightly less than that of the mould.

Prior to final positioning of the mandrel as shown in FIG, 17, theplunger 166 pushes the void ring 121 away from the end of the cementcylinder 170, to the position shown in FIG. 17. In the latter stages ofthis movement, the flange 142 passes beyond the end of the cement,allowing expansion of the mandrel to occur. In this latter stage also,the valve arm 163a of the nonrotating mandrel strikes the arm 164 of therotating mould, causing the mandrel to rotate at the same speed as themould, and also opening the valve 163 to allow pressurized air to passvia conduits 162 from the interior of shaft 144 to air cylinder 161. Aircylinder 161 then expands, causing expansion of the outer surface of themandrel into contact with the interior of the cement cylinder 149, withconsequent expansion of the flange 142 into the space between the end ofthe cement cylinder and the void ring 121. Rotation of the mould is thenstopped, the parts being then positioned as shown in FIG. 17. Thecarriage 148 is then withdrawn, carrying the mandrel which in turncauses the cement cylinder 149 together with liner 170 to be slid(without dimensional change) from the mould. During this movement,flange 142 is maintained in the expanded condition and acts asextracting means engaging the thickened end of the liner and preventingthe cement cylinder and liner from sliding off the mandrel, and the clip175 holds the liner edges together.

FIGS. 18a and 18b show further means for slitting the cylinders andlaying out the resultant sheets, particularly for use in connection withthe apparatus of FIGS. 13 to 17. As shown, a series of similar pallets185 is provided, each having a lug 186 extending along one edge andadapted to engage in the recesses of edge pieces 171 of the liners. Asshown in FIG. 18b just outside the lug 186 and parallel thereto issituated one strand 187 of a wire band saw, which saw is verticallymovable relative to the pallet.

At each side of the pallets 185 is situated a vertical wall 189, havinga horizontal tapered upper edge suitably spaced to engage an annularrecess in the support shafts 144, of the appropriate end of mandrel 140,as shown in FIG. 18a. The walls 189 are adjustable in height by raisingmeans indicated at 190. When raised to their full height, the walls 189are capable of forming a steam curing chamber when combined with theparts shown in FIG. 18c. These parts comprise a top 191 suit abie forcovering the space between walls 189, and hinged end pieces 192 sized toclose the ends of the chamber.

In operation of the apparatus shown in FIG. 18, a first pallet 185 islaid between the walls 189 while the walls are in a lowered position,After a cement cylinder has been removed from the mould on the mandrel140, the mandrel is positioned directly above the band saw 187, with itssupport shafts 144, 145 supported at one end of walls 189, and with thegap in the mandrel surface and the liner edge portions positioned asshown in FIG. 18b. The clip is removed from the liner, and one edgeportion only of the liner is then re-attached to the mandrel by twoU-clips shown at I94, which have opposed flanges engaging the inside ofthe mandrel and the outside of the edge piece 171, one clip being usedat each end of the mandrei. The other edge portion 171 is engaged by lug186 as shown in FIG. 18b. The wire band saw 187 is then operated andmoved vertically to cut through the cement cylinder, the saw passing between the liner edge portions, by deflecting the edge portions wherethey abut, and also pasing through the gap between the longitudinaledges of plate 141. The cylinder and liner are then rolled out flat onthe pallet I85, with one end of the liner remaining attached to thepallet and the other end remaining clipped to the mandrel untilunrolled. During this unrolling operation, the mandrel support shafts144 and 14S roll and slide along the top edges of walls 189, themovement of the mandrel being such as to stretch and roll the sheet ofcemerit,

After a first sheet of cement has been laid out as described, a furtherpallet 1850 is laid thereon, and the next sheet is laid on this furtherpallet, this operation being repeated until a stack of palletsinterposed with sheets of cement and liners of produced. As successivesheets of cement and pallets are laid down, the walls 189 are raisedsimultaneously by raising means 190, so as to remain at the same heightas each other and so as to continue to support the mandrel during eachunrolling operationv After a stack of about twenty cement sheets havebeen laid out in this manner, and with the walls 189 raised to theirmaximum height, a curing chamber is formed by arranging on these wallsthe top 191 and end pieces 192 as shown in FIG. 186, The stack of cementsheets is then steam cured to harden them, after which the sheets areremoved and the pallets and liners recovered for re-use. It may be notedthat, unlike in the first embodiment, the sheets are cured and hardenedwithout being moved from the position in which they are laid out.

The liners may be re-used by being bent up around a suitable mandrel,and cleaned on their external surfaces before being inserted into themould. After inser

1. A PROCESS FOR PRODUCING A FIBREINFORCED CEMENT SHEET COMPRISING THESTEPS OF: A. FEEDING A CEMENT MIXTURE TOGETHER WITH REINFORCEING FIBERSINTO A ROTATABLE HORIZONTAL MOULD, B. SPINNING THE MIXTURE IN THE MOULD,DISTRIBUTING THE MIXTURE AROUND THE MOULD WALLS, AND REMOVING WATERTHEREFROM, TO FORM A SOFT, HOLLOW CYLINDER, C. INSERTING A CYLINDRICALMANDREL INTO THE SAID CEMENT CYLINDER TO ENGAGE THE INNER SURFACE OFSAID CYLINDER, D. SLIDING THE CEMENT CYLINDER FROM THE MOULD WITHOUTDIMENSIONAL CHANGE BY WITHDRAWING SAID MANDREL WHILE HOLDING SAIDCYLINDER ON THE MANDREL AND SUPPORTING THE INNER SURFACE OF THE SOFTCEMENT CYLINDER BY MEANS OF SAID MANDREL, E. DIVIDING SAID CEMENTCYLINDER LONGITUDINALLY, AND F. PROGRESSIVELY REMOVING THE CEMENTCYLINDER FROM THE MANDREL AS A SHEET BY INITIALLY PLACING ONE EDGE OFTHE SHEET ON A RECEIVING SURFACE DISPOSED BENEATH SHEET FROM DREL ANDSUBSEQUENTLY UNROLLING THE CEMENT SHEET FROM THE MANDREL BYSIMULTANEOUSLY ROTATING THE MANDREL AND CAUSING RELATIVE MOVEMENTBETWEEN TH MANDREL AND THE SAID RECEIVING SURFACE TRANSVERSE TO THEMANDREL AXIS.
 2. A process according to claim 1, further including thesteps of applying a vacuum to ports in the cylindrical surface of themandrel after the mandrel has been inserted into the cylinder to causethe cylinder to cling to the surface of the mandrel, and releasing saidvacuum prior to or during the step of removing the cement cylinder fromthe mandrel.
 3. A process according to claim 1, further comprising theinitial step of inserting a cylindrically curved flexible sheet ofmaterial into the mould so that the material forms a liner conforming tothe mould surface and has adjacent edges extending longitudinally of themould, said liner defining the outer surface of said cement cylindersubsequently formed in the mould, and wherein said step of holding saidcylinder on the mandrel is performed by moving extracting meansconnected to said mandrel into position to engage the liner and to causethe liner to move with the mandrel when the mandrel is withdrawn, thesaid adjacent edges of the liner being connected together during thestep of withdrawing the mandrel from the mould and being disconnectedprior to dividing the cement cylinder longitudinally.
 4. A processaccording to claim 1, wherein said step of dividing the cement cylinderlongitudinally is performed by longitudinally slitting the cementcylinder while it is supported by the mandrel outside the mould.
 5. Aprocess according to claim 1, wherein said cement cylinder is formed inthe mould with a longitudinal discontinuity caused by an inwardlyprojecting member extending longitudinally of the mould, and wherein thestep of dividing the cement cylinder longitudinally involves splittingthe cement cylinder along said discontinuity.
 6. A process according toclaim 1, wherein said fibres includes asbestos fibres.
 7. A processaccording to claim 3, wherein, prior to feeding said cement mixture intothe mould, particles of stone having an average size of between 1/2 inchand 1/64 inch are distributed in the mould to form a cylinder of saidparticles within the liner, said cement mixture being subsequently fedinto the mould and distributed within the mould to form a cementcylinder within said cylinder of particles and adhering thereto.